Q&A: How does Flat Armor and Percent Armor Penetration stack?

This is one question I get emailed frequently, so when the opportunity comes to intertwine a commonly asked question and a red post; I just have to take it. This weeks question deals with the different between Flat Armor and Percent Armor penetration as well as how the two interact with each other. Keep this small synopsis in mind next time you are gearing out your AD carry for maximum effectiveness.

Flat Armor Penetration: Flat armor penetration is applied directly to your attacks and physical abilities. These attacks will deal damage as though your target has less armor than he really does, but will never bring an opponent below 0 armor. For example, if a target has 20 armor and you have 5 armor penetration, you would deal damage as though the target had 15 armor. If you had 50 armor penetration against the same target, you would deal damage as though he had 0 armor.

Percent Armor Penetration: Percent armor penetration applies its benefits after flat armor penetration. For example if you fight a target with 120 armor while you have 20 flat armor penetration and 40% armor penetration, you would deal damage to your target as though he had 60 armor.

Only armor reduction can take effective armor to negative values and deal bonus damage

I read through most of the red posts the occur on the forum every day and try to cherry pick the most informative ones. 75% of them are funny, casual, or jokes but a good amount of them give hints into chacacter development, rewords, buffs/nerfs, and upcoming content. Couple this with twitter and other social media that rioters post on and this is where I get most of my news.